1. Field of the Invention
The invention relates to composite structures comprised of carbon fibers and a resin matrix and a means for enhancing the bonding characteristics of the fiber and resin while improving the flexural strength of such composites. The term carbon fibers is used herein in the generic sense and include both graphite fibers and amorphous carbon fibers in filament, tow, staple yarn or roving configuration.
2. Description of the Prior Art
Carbon fiber/resin matrix composite structures often exhibit low inerlaminar composite shear strengths, which is a measure of interfacial bonding between the fibers and the resin, because of relatively poor adhesion between the fibers and the matrix.
In the past, improved bonding has been accomplished by coating the surfaces of the fibers with various metals (e.g., tantalum), metal carbides (e.g., whiskers of silicon carbide) and nitrides. These coating processes are inherently expensive and it is difficult to control the thickness and homogeneity thereof. Further, these coatings do not enhance the flexural strength of the final composite structure.
Additionally, various surface treatments known in the art have been applied to the fibers to improve their bonding characteristics. Such treatments, however, are conventionally oxidative or corrosive etches of various kinds, and include treatment with air, ozone, concentrated nitric acid, chromic-sulfuric acid, nitrogen dioxide and the like. Although these treatments do tend to improve composite shear strength, they proceed to the internal portions of the fibers and actually decrease other fiber and/or composite physical properties. Furthermore, surface treatments of this nature are expensive, relatively inconvenient to apply to the fibers, often pollute the atmosphere, and can be hazardous to the health of workers in the area where the treatment is taking place.